1. Field of the Invention
This invention relates to a dialing tone generator for automatically converting a telephone number into a dialing tone signal and sending it to a push button telephone set, thereby calling a telephone of a second party.
2. Description of the Prior Art
A dialing tone signal of a push button telephone set generally consists of a composite signal containing one of four low-frequency signals of 697 Hz, 770 Hz, 852 Hz, and 941 Hz and one of four high-frequency signals of 1209 Hz, 1336 Hz, 1477 Hz, and 1633 Hz.
In this case, each frequency signal is produced by dividing an oscillation frequency signal of an oscillator. In general, to obtain signals of different frequencies as described above, an oscillator with an extremely higher frequency than that of the dialing tone signal is required.
Table 1 below shows a frequency dividing ratio (N), a frequency (F/N), and a frequency error (.DELTA.F%) of a known dialing tone generator for dividing an oscillation frequency signal of 3 MHz (precisely 3,579,545 Hz) to obtain dialing tone frequency signals as indicated by R1 to R4 and C1 to C4, respectively.
TABLE 1 ______________________________________ FREQ 3.579545 MHz (Hz) F/N N .DELTA.F % ______________________________________ Low R1 697 699.1 160 .times. 32 +0.30 Frequency R2 770 766.2 146 .times. 32 -0.49 Group R3 852 847.4 132 .times. 32 -0.54 R4 941 948.0 118 .times. 32 +0.74 High C1 1209 1215.9 92 .times. 32 +0.57 Frequency C2 1336 1331.7 84 .times. 32 -0.32 Group C3 1477 1471.9 76 .times. 32 -0.35 C4 1633 1645.0 68 .times. 32 +0.73 ______________________________________
Since this known dialing tone generator uses an extremely high-frequency signal such as 3 MHz, frequency errors with respect to the frequency signals as indicated by R1 to R4 and C1 to C4 are very few, but many frequency dividing stages are required. As a result, a bulky apparatus with great power consumption is required, and when a battery is used, a battery life will be shortened.
In addition, when the conventional dialing tone generator is incorporated in a small-sized electronic appliance, e.g., a portable calculator or a wristwatch, a high-frequency oscillator such as a quartz oscillator of 3 MHz and many frequency dividing stages are additionally required. For this reason, current consumption is very increased to thereby shorten a battery life, and a device itself must be enlarged to allow space for the electronic circuit. Furthermore, if a typical oscillator (oscillation frequency F=32.768 [KHz]) incorporated in a normal electronic wristwatch and the like is utilized, a dialing tone frequency error caused by the oscillator falls outside a range of .+-.1.5% as an allowance value. More specifically, Table 2 below shows the case where a frequency of 32.768 KHz is utilized.
TABLE 2 ______________________________________ FREQ 32.768 KHz (Hz) F/N N .DELTA.F % ______________________________________ Low R1 697 697.2 47 +0.029 Frequency R2 770 762.0 42 -1.04 Group R3 852 862.3 38 +1.21 R4 941 936.2 35 -0.51 High C1 1209 1213.6 27 +0.38 Frequency C2 1336 1310.7 25 -1.89 Group C3 1477 1489.5 22 +0.85 C4 1633 1638.4 20 +0.33 ______________________________________
As compared with various parameters illustrated in Table 1, although a dividing ratio (N) is extremely small, conversely, a frequency error is large. For instance, an error becomes 1.89% with respect to a dialing tone frequency of 1336 Hz, and hence is not acceptable. In addition, errors with respect to dialing tone frequencies of 770 Hz and 852 Hz are extremely large.
Furthermore, in the conventional dialing tone generator, since frequency signals of dialing tone signals must be sine waves, a complex circuit for converting each frequency signal into a sine wave is required.
For example, U.S. Pat. No. 4,563,548 to Misherghi et al., issued on Jan. 7, 1986, describes a technique for obtaining sequentially increasing/decreasing data by an ALU (Arithmetic Logic Unit) and an accumulator and for converting digital data obtained by the ALU and the accumulator into a sine-wave signal by a D/A (Digital to Analog) converter so as to obtain a dialing tone signal. In addition, U.S. Pat. No. 4,087,638, issued to Hayes et al. on May 2, 1978, also describes another technique for converting sequentially increasing/decreasing digital data into a sine-wave signal by a D/A converter to obtain a dialing tone signal.
As described above, in these techniques wherein sequentially increasing/decreasing digital data is supplied to a D/A converter to obtain a dialing tone signal of a sine wave, different data must be prepared for the respective frequencies. Therefore, a circuit arrangement becomes very complex and fails to provide a compact device.